The continuing transformation of the health care system into one characterized by competing managed care health plans has raised concerns regarding the quality of care that results in this new environment. It is for this reason that policy makers, purchasers, the media, and private organizations have devoted significant resources to measuring health plan quality. The goal of this project is to examine the level of, and changes in, health plan quality in this new environment. We will analyze the relationship between quality (and changes in quality) and a variety of market and plan factors. Our measures of quality are based on the Health Plan Employer Data and Information Set (HEDIS) and the Consumer Assessment of Health Plans Survey (CAHPS), which, though imperfect, are the most widely used indicators of non-financial aspects of health plan performance. The project will make use of a comprehensive panel data set capturing plan quality, including data not publicly reported. The project will also incorporate unique data on the overlap in plans' provider networks because such overlap could affect health plan performance. Identification of factors that promote quality will help policy makers identify where competitive strategies may fail to improve quality and where other policy actions may be needed. The nature of the data surrounding managed care and the consumers' health care bill of rights could be affected by evidence regarding HMO quality and changes in that quality over time.